The invention relates to a method for operating an internal combustion engine with variable valve drive and to an arrangement for carrying out the method.
Internal combustion engines refer to combustion engines which are used as drives in motor vehicles. Internal combustion engines are heat engines which, by way of a combustion process, convert chemical energy of a fuel into mechanical energy. As combustion chambers, cylinders are provided, in which in each case one piston is guided.
Valves are used for the admission of the fuel-gas mixture, which is to be ignited, into the cylinders. In the case of diesel engines, inlet valves are used for the admission of pure air. Fuel is injected by way of injection nozzles. In the case of gas engines, a pre-compressed air-gas mixture is introduced into the cylinders via inlet valves. For the control of the valves, a mechanism is provided which is referred to as valve controller or valve drive. By opening and closing the valves, the charge exchange of the internal combustion engine is controlled. In general, valves are opened by a camshaft, which is driven by a crankshaft, via a tappet. Here, “charge exchange” refers to the exchange of the mixture contained in the cylinder.
A variable valve drive or a variable valve controller makes it possible to realize a variable opening time, a variable valve lift and a variable spread, and reduces the fuel consumption through the influencing of the charge exchange work, the mixture preparation and the combustion. The load of the engine can be controlled by way of a variable valve drive of said type. For this purpose, so-called valve control management (VCM) is required. It is thus possible for the load and thus the power of the engine to be controlled, and thus also regulated, by influencing the opening and closing times of the individual cylinders.
It must be taken into consideration that, in the low-load range, the handling of the variable valve drive system is impeded because the valve must be braked during its upward movement and closed again in order to realize short opening times. This comes at the cost of precision, and is technically not practicable for very short valve opening times.
A further problem consists in the determination of the air mass in the cylinder. In the case of a variable valve drive system, an air mass sensor is installed, but this exhibits only inadequate accuracy in the presence of the low air mass flows that prevail in the low-load range.